The present invention relates to a power switchgear for operating an electric circuit.
FIGS. 1-4 represent one example of a conventional power switchgear. In the drawings, 1 denotes a mounting formed of a metallic steel plate, which is provided with a plurality of fitting holes 1a used to arrange a power switchgear body therethrough; 2 denotes a base formed of an insulating material, which is fixed on the mounting plate 1 with a screw 3; 4 denotes a fixed core having a silicon steel plate laminated thereon. An operating coil 5 is installed on the fixed core 4, and further a leaf spring 6 is arranged in a gap with the mounting plate 1 as a shock absorber. Numeral 7 denotes a moving core arranged opposite to the fixed core 4, which is pulled toward the fixed core 4 when the operating coil 5 is conducting; 8 denotes a cross bar formed of an insulating material, which is coupled to the moving core 7 through a pin 9; 10 denotes a trip spring arranged between the cross bar 8 and the mount 1, which normally lifts the cross bar 8 so that a main circuit of the power switchgear is maintained open; 11 denotes a moving contact-maker provided with a moving contact 11a, which is inserted in a holding hole 8a provided on the cross bar 8 and urged by a pressure spring 12; 13 denotes a fixed contact-maker provided with a stationary contact 13a opposite the moving contact 11a. The fixed contact-maker 13 is fixed on a terminal 15 with a screw 14, and the terminal 15 is fixed to the base 2 with screws 16, 17. Numeral 13b denotes an arc runner connected electrically to the fixed contact-maker 13, which can be unified with the fixed contact-maker 13; 14 denotes a terminal screw connected to a main circuit wire, which is fitted to the terminal 15; 19 denotes an arc box formed of an insulating material, which is fixed on the base 2 with a screw 20. The arc box 19 includes a hole 19a through which gas is discharged, a ceiling part 19b and a side plate 19c. Numeral 21 denotes a deion grid arranged in a shape as in FIG. 4 and made of a magnetic material; 22 denotes a commutating electrode, which is fixed on the ceiling part 19b of the arc box 19. The stationary contact 13a and the moving contact 11a are formed in the internal space of an arc extinguishing chamber.
The operation of the power switchgear as thus arranged will now be described. When a voltage is impressed on the operating coil 5 with the main circuit shown in FIG. 1 open, a magnetic flux is generated between the fixed core 4 and the moving core 7, and the moving core 7 is moved toward the fixed core 4 against the force of the trip spring 10. In this case, the cross bar 8 coupled to the moving core 7 moves downwardly, the moving contact 11a of the moving contact-maker 11 comes in contact with the stationary contact 13a of the fixed contact-maker 13, and a predetermined pressure is applied by the pressure spring 12 to open the main circuit. Next, when the operating coil 5 is deenergized, the moving core 7 moves away from the fixed core 4 by the force of the trip spring 10, and the cross bar 8 also moves with the moving core 7. Therefore, the cross bar 8 returns to the state shown in FIG. 1, and the moving contact 11a of the moving contact-maker 11 and the stationary contact 13a of the fixed contact-maker 13 are separated. In the process, an arc is generated between the moving contact 11a and the stationary contact 13a at a portion indicated in FIG. 1 at A. The movement of the arc until the current is interrupted after it is generated is illustrated for only one side in FIG. 5, as the arc extinguishing chamber B in FIG. 1 is symmetrical. FIG. 5a represents the state wherein the stationary contact 13a and the moving contact 11a are closed. When the stationary contact 13a and the moving contact 11a are opened when the operating coil 5 is conducting, an arc 23 is generated, as shown in FIG. 5b, between the stationary contact 13a and the moving contact 11a. The contact opening distance gets larger as time passes, up to the maximum distance. The arc 23 is driven and expanded, as shown in FIG. 5c, by the current flowing in the moving contact-maker 11 and the fixed contact-maker 13 and the deion grid 21, and one end of the arc 23 is transferred, as shown in FIG. 5d, from the surface of the stationary contact 13a to the arc runner 13b. Then, there occurs a dielectric breakdown between a tip of the arc 23 shown in FIG. 5d and a portion of the arc runner 13b indicated at A, and an end of the arc 23 is transferred to the portion of the arc runner 13b indicated at B in FIG. 5e. The other end of the arc 23 is transferred, as shown in FIG. 5f, from the stationary contact 11a to the commutating electrode 22 and the arc 23 is extinguished between the deion grids 21. Thus, the current is cut off completely. As noted, the power switchgear has a commutating electrode 22 positioned on the rear side of the moving contact 11, and therefore a long time is required for one end of the arc 23 to transfer from the moving contact 11a to the commutating electrode 22. The shortcoming that the expensive moving contact 11a is subject to wear is consequently unavoidable.